I agree on tye's last comment; font & color changes specified by the poster are rather unwanted by most users. In talking with those that are prepping for the massive improvements to CSS usage on the site, however, I would suggest that it's rather trivial to add new html-like tags that can be used by someone with their own CSS to markup pages in their own way.

For example, <spoiler>...</spoiler> would be translated to <div class="spoiler">...</div> (or span, depending on body or character context), and the default style sheet could be set to make these black text on black bg, but still allow a user to change that with their own specified stylesheet.

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Dr. Michael K. Neylon - mneylon-pm@masemware.com || "You've left the lens cap of your mind on again, Pinky" - The Brain
"I can see my house from here!"
It's not what you know, but knowing how to find it if you don't know that's important